This invention relates generally to amplifiers and more particularly to amplifiers with forward feed.
In a basic feed-forward system, such as described, for example, in U.S. Pat. No. 4,146,844 issued to P. A. Quinn on Mar. 27, 1979, samples of the input and output signals of a main amplifier are adjusted in amplitude and algebraically combined at a first summing node to produce a sample of the error (e.g., distortion and noise) introduced into the output signal by the main amplifier. This error sample is adjusted in amplitude and fed forward to a second summing node where it is algebraically combined with the output of the main amplifier to cancel the error in the output signal. In a typical feed-forward system, the error sample is adjusted in amplitude by amplification in a subsidiary amplifier which itself introduces noise and distortion. As a result, the output signal is accompanied by an output error component caused by the subsidiary amplifier which is of magnitude NG.sub.2, where G.sub.2 is the complex gain factor of the subsidiary amplifier, and N includes both the equivalent input noise voltage of the subsidiary amplifier as well as any other signals (referred to its input) arising from the non-linearity of the subsidiary amplifier. This output error component can in many cases be greater than the distortion and noise that the feed-forward loop was designed to eliminate.
U.S. Pat. No. 3,471,798, issued to H. Seidel on Oct. 7, 1969, discloses a multiple-loop feed-forward-compensated amplifier in which the feed-forward loop is iterated by considering the entire feed-forward amplifier to be the "main" amplifier which is to be feed-forward compensated. This modification provides compensation for error introduced by the subsidiary amplifier, but requires the addition to the circuit of a second subsidiary amplifier which itself can cause an intolerable output error component.